Malis Vadim, Sinha Usha, Sinha Shantanu
Department of Physics, University of California, San Diego, San Diego, CA, United States.
Department of Radiology, University of California, San Diego, San Diego, CA, United States.
Front Physiol. 2020 Dec 3;11:600590. doi: 10.3389/fphys.2020.600590. eCollection 2020.
3D strain or strain rate tensor mapping comprehensively captures regional muscle deformation. While compressive strain along the muscle fiber is a potential measure of the force generated, radial strains in the fiber cross-section may provide information on the material properties of the extracellular matrix. Additionally, shear strain may potentially inform on the shearing of the extracellular matrix; the latter has been hypothesized as the mechanism of lateral transmission of force. Here, we implement a novel fast MR method for velocity mapping to acquire multi-slice images at different % maximum voluntary contraction (MVC) for 3D strain mapping to explore deformation in the plantar-flexors under isometric contraction in a cohort of young and senior subjects. 3D strain rate and strain tensors were computed and eigenvalues and two invariants (maximum shear and volumetric strain) were extracted. Strain and strain rate indices (contractile and in-plane strain/strain rate, shear strain/strain rate) changed significantly with %MVC (30 and 60% MVC) and contractile and shear strain with age in the medial gastrocnemius. In the soleus, significant differences with age in contractile and shear strain were seen. Univariate regression revealed weak but significant correlation of and and indices to %MVC and correlation of and indices to force. The ability to map strain tensor components provides unique insights into muscle physiology: with contractile strain providing an index of the force generated by the muscle fibers while the shear strain could potentially be a marker of lateral transmission of force.
三维应变或应变率张量映射全面地捕捉区域肌肉变形。虽然沿肌纤维的压缩应变是所产生力的一种潜在度量，但纤维横截面中的径向应变可能提供有关细胞外基质材料特性的信息。此外，剪切应变可能潜在地反映细胞外基质的剪切情况；后者被假设为力横向传递的机制。在此，我们实施一种用于速度映射的新型快速磁共振方法，以获取不同最大自主收缩（MVC）百分比下的多层图像，用于三维应变映射，以探究年轻和老年受试者队列在等长收缩时跖屈肌的变形情况。计算了三维应变率和应变张量，并提取了特征值和两个不变量（最大剪切应变和体积应变）。内侧腓肠肌的应变和应变率指数（收缩和平面内应变/应变率、剪切应变/应变率）随MVC百分比（30%和60%MVC）有显著变化，收缩和剪切应变随年龄变化。在比目鱼肌中，收缩和剪切应变随年龄有显著差异。单变量回归显示， 和 指数与MVC百分比之间存在弱但显著的相关性， 和 指数与力之间存在相关性。映射应变张量分量的能力为肌肉生理学提供了独特的见解：收缩应变提供了肌肉纤维产生力的指标，而剪切应变可能潜在地是力横向传递的标志物。
